Wisconsin has the fortunate distinction of not having any “reported” cases of glyphosate-resistant weeds (other than volunteer corn). Unfortunately, glyphosate-resistance will happen in Wisconsin. Since it’s just a question of when, our hope is to delay and limit its impact.

 

What practices will reduce the risk of herbicide resistance? First, we need to understand herbicide resistance. The risk of resistance is really just a numbers game. For each herbicide and weed species, a certain probability exists that a naturally occurring resistant plant may be in a population, which could lead to a resistance problem. For example, the probability of naturally occurring resistance seems to be lower with glyphosate than for some other herbicide modes of action. For some weeds like horseweed or waterhemp, the probability seems to be higher than for other weeds. The actual chance of finding one of these resistant plants is where the “numbers game” begins. The more weeds that are sprayed, the greater the risk. If you spray a billion weeds, there is a greater chance one plant might be resistant than if you spray a million weeds. It’s just like the lottery. Odds of winning increase as you buy more tickets, except you don’t want to “win” resistance.

 

To reduce the risk of resistance, we need to think about how we can reduce the number of weeds sprayed with glyphosate. We can accomplish this in a couple different ways. We can rotate glyphosate use with other herbicides so we are not spraying weeds with glyphosate each year. We can also use other herbicides in the same season as glyphosate. Options are to use a preemergence (PRE) herbicide before glyphosate or to tank mix another herbicide with glyphosate. The key for this strategy to be effective is that the second herbicide(s) needs to have activity on all of the target weeds. We should also maintain good weed management programs so there are fewer weeds that are sprayed when glyphosate is used.

 

An 8-year study at Arlington can help to show how these strategies can lower the risk of resistance. Over the 8-year study, giant foxtail and common lambsquarters were counted to determine the number of seedlings sprayed with glyphosate. Four of the herbicide programs are summarized in Table 1, which shows the total number of seedlings sprayed when adjusted to a 75 acre field size. The probability of resistance was determined based on weed characteristics and a 1 in 10 billion chance of glyphosate resistance. The relative risk of resistance compares the probability of resistance to the program with lowest probability, which was giant foxtail in the Dual PRE before glyphosate program. For example, spraying glyphosate alone had a 47x greater risk of selecting a resistant giant foxtail and 35x greater risk of selecting a resistant common lambsquarters than when Dual was used PRE before glyphosate. 

 

 

Table 1. Number of giant foxtail and common lambsquarters plants sprayed with glyphosate during an 8 year field experiment and adjusted to a 75 acre field size and the relative risk of glyphosate resistance compared to the lowest risk program (Dual II Magnum followed by glyphosate).

                                                                                                                                                

	Giant foxtail		Common lambsquarters
Program	Millions	Relative risk	Millions	Relative risk
Dual II Magnum PRE	8	1	86	11
before glyphosate POST
Glyphosate POST/non-glyphosate	83	10	62	8
annual herbicide rotation
Glyphosate POST once	388	47	280	35
Glyphosate POST twice	303	37	186	24

 As we have stated numerous times, glyphosate and glyphosate-resistant crops are valuable technologies. We should all promote and use practices to protect their long-term value. Because resistance is a numbers game, remember that billions of weeds are being sprayed each year with glyphosate on millions of acres, increasing the odds of resistance. We need to tilt the odds in our favor by using rotations, sequential treatments, and tank mixtures to lessen the risk.